The present invention relates to the determining of a composition of a gas mixture comprising an amount of one or more noble gases.
Gas detection, in general, is accomplished either by using optical absorption or by scattering of light For absorption measurements, primarily the infrared (IR) spectral region is used, where the excitation of molecular vibrations contributes to the dipole moment of a molecule. Atomic gases, such as the noble gases, do not exhibit IR absorption. Scattering of light occurs as a consequence of the electronic polarizability of the electron cloud around atoms and molecules. The law of photometric summation applies, so that the total energy scattered by N molecules is just N times the energy scattered by one molecule.
Most incident photons are scattered by the sample with no change in frequency in a process known as Rayleigh scattering. Rayleigh scattering occurs from molecular as well as atomic species. However, with a small probability the scattered photons have frequencies f.sub.0 +/-f.sub.1, where f.sub.0 is the frequency of the incident photon and f.sub.1 is the frequency of a molecular vibration. This process is called Raman scattering. The modification of the scattered photons results from the incident photons either gaining energy from or losing energy to the vibrational or rotational motion of the molecule. Since complex molecules exist in a number of different rotational and vibrational states (depending on the temperature), many different values of f.sub.1 are possible. Consequently, the Raman spectrum of a Raman-active gas will consist of a large number of scattered lines. Simple diatomic molecules like oxygen, O.sub.2, or nitrogen, N.sub.2, have just one Raman line.
To enhance the observation of the radiation at f.sub.0 +/-f.sub.1, the scattered radiation is observed perpendicularly to the incident beam. To provide high intensity incident radiation and to enable the observation of lines where f.sub.1 is small (due to rotational changes), the source of a Raman spectrometer is normally chosen as a monochromatic visible laser. The scattered radiation can then be analyzed by use of a scanning optical monochromator with a photomultiplier tube or another suitable photo detector.
Noble gases have stable electron configurations and thus do not easily gain or lose electrons and rarely share them with other elements. Therefore, noble gases exist only as mono-atomic species that do not have any vibrational states and consequently do not give rise to Raman scattering, so that noble gases are generally Raman non-active.
A possibility for noble gas detection as known in the art is the resonanceionization mass spectrometry (RIS). However, RIS can be applied to the inert or noble gases only with great difficulty due to the short wavelength required for the first excitation step.
Xenon (Xe), as a noble gas, has been investigated as an anaesthetic gas and has been proved as a possible anesthesia means substantially free of side effects and innocuous for the earth atmosphere and environment. Clinical tests for medical applications are underway e.g. in Germany.
A general difficulty with applying xenon as a component of a respiration gas is the required measuring technique for a quantitative monitoring of the xenon concentration. Respiration gas monitors as known in the art are generally based on infrared absorption and allow detecting CO.sub.2, N.sub.2 O, and other commonly applied volatile anesthetics (e.g. halothane, enflurane, isoflurane, desflurane, and sevoflurane). Oxygen (O.sub.2) is detected with separate measuring cells, e.g. making use of the paramagnetic properties of oxygen. Nitrogen (N.sub.2) cannot be detected with those monitors. As pointed out above, noble gases, such as xenon, are not detectable by measuring methods applying infrared and can only be detected by mass spectrometry, which, however, requires expensive monitors, and are thus normally not applicable for standard applications in hospitals.